In recent years, technological innovation in automobiles has been outstanding responding to the trend of protection of the earth environment. As a part of the technology, hybrid cars are already commercially available. Further, an automobile with stop-idling function, which temporarily stops the engine under certain conditions and automatically restarts the engine, has been developed and sold in the market.
Restarting the engine of such an automobile with stop-idling function requires a large current, thus causing the voltage of the in-vehicle power supply to be momentarily lowered. This phenomenon may cause an in-car device such as a car navigation system controlled by a microprocessor to be automatically reset. When the in-car device automatically restarts, the user needs to reset his/her desired settings. To solve the problem, the suggestion is made that a backup circuit including a battery or electric double layer capacitors be mounted on an automobile, which is already available in the market.
An electric double layer capacitor is a key component supporting the technology. The capacitor is generally used in conjunction with a lead storage battery (referred to as “battery” hereinafter) which is conventionally used. As a method of charging an electric double layer capacitor, using braking energy is proposed in Japanese Patent Unexamined Publication No. H11-122709, for example.
Meanwhile, Japanese Patent Unexamined Publication No. H02-259276 proposes that electric double layer capacitors be series-connected to the battery, and the starter be driven by them to start the engine, thereby reducing the load to prolong the battery life.
However, in a conventional engine starter, electric charge charged while the vehicle is operating remains unchanged in the electric double layer capacitors while the vehicle is non-operating. A non-operating state continuing in such circumstances places a heavy load on the capacitors, thereby causing the life to be shortened. From such a viewpoint, electric charge charged in the capacitors is preferably discharged while the vehicle is non-operating. Meanwhile, restarting after stopping idling requires a current of tens of amperes at approximately 12 V. Consequently, when discharging the capacitors as described above, frequent restarting places a heavy load on the lead storage battery to impair the effect of using electric double layer capacitors, resulting in significantly decreasing the battery life.
As a method of charging plural series-connected electric double layer capacitors, “balanced circuit method” has been employed conventionally. In this method, balance resistances R101, R102, R103, Rn with the same resistance value are parallel-connected to electric double layer capacitors C101, C102, C103, Cn, respectively, as shown in FIG. 3. This arrangement causes the bias voltage to each capacitor to be balanced, thereby applying substantially the same voltage to each capacitor.
In this balanced circuit method, the balance resistance is always connected to each electric double layer capacitor. The entire electric circuit thus forms a closed loop, and as a result, electric energy accumulated in each capacitor is gradually lost due to discharge. Accordingly, the balance resistance needs to be parallel-connected and disconnected to and from an electric double layer capacitor using such as a transistor. However, if discharge by the balance resistance is completely suppressed, a heavy load is placed on the capacitors, resulting in shortening the life.
When using a car navigation system or watching TV while the ignition power is off and only the accessory power is on after the engine stops, their operating current is supplied from the battery. Even if the automobile is in a complete stop, a dark current is always supplied to the receiving unit of a remote controller for locking and unlocking, a clock, and other devices. All of the dark currents are supplied from the battery. Consequently, the voltage of the battery is gradually lowered. The voltage deficiency is compensated for by electric energy accumulated in the electric double layer capacitors, thereby the voltage of the capacitors is gradually lowered as well. If both voltages of the battery and capacitors drop, starting the engine becomes difficult.